Differences in Physical Match Performance and Injury Occurrence Before and After the COVID-19 Break in Professional European Soccer Leagues: A Systematic Review

Background Due to the COVID-19 pandemic, matches and soccer-specific training were suspended for several weeks, matches after resumption were congested, and substitutions per team and game increased from three to five. Objective The aim of this review was to examine possible differences in physical match performance and injuries between before and after the COVID-19 induced break of matches and training in professional male European soccer leagues during the 2019/2020 season. Methods A systematic search identified all scientifically peer-reviewed publications involving elite male soccer players competing in the European leagues which reported physical match performance variables such as total running distance and running distance at different speed zones and/or injury parameters pre- and post-COVID-19 induced break. Results In total, 11 articles were included, which were coming from German Bundesliga, Polish Ekstraklasa, Croatian HNL, Spanish La Liga, and Italian Serie A. In all studies investigating the German Bundesliga, most parameters of physical match performance remained unaffected (0.08 ≤ p ≤ 0.82; − 0.15 ≤ ES 0.15), while studies investigating the Polish Ekstraklasa (p ≤ 0.03; − 0.27 ≤ ES − 0.18), Croatian HNL (p ≤ 0.04; − 1.42 ≤ ES ≤ 1.44), Spanish La Liga (p ≤ 0.017; − 0.32 ≤ ES ≤ 5.5), and Italian Serie A (p ≤ 0.014; − 1.01 ≤ ES 0.24) showed a decrease in most parameters of physical match performance after the COVID-19 break. Injury rates were only investigated by studies targeting the German Bundesliga and Italian Serie A. In the majority of studies (3 out of 4 studies), there occurred no difference in injuries between pre- and post-COVID-19 break (p > 0.05; ES = N/A). Conclusion Results indicate that Bundesliga teams maintained physical match performance during the 9-weeks break in matches and 3-weeks break in group training, whereas a longer match and group training interruption up to 15 weeks and 8 weeks, respectively, in the other leagues appeared to lead to a decreased physical match performance. Regarding injuries, we speculate that the increase in substitutions from 3 to 5 substitutions per game might prevent an increase in injury occurrence during matches. The underlying studies’ results provide hints for possible upcoming unexpected interruptions with respect to optimal physical preparations for the resumption of matches and a congested schedule to maintain physical match performance, or for possible rule changes such as 5 instead of 3 substitutions to avoid physical overload during congested match schedules. Supplementary Information The online version contains supplementary material available at 10.1186/s40798-022-00505-z.


Search strategy
7 Present the full search strategies for all databases, registers and websites, including any filters and limits used. 6-7, Suppl. C

Selection process
8 Specify the methods used to decide whether a study met the inclusion criteria of the review, including how many reviewers screened each record and each report retrieved, whether they worked independently, and if applicable, details of automation tools used in the process.
7 Data collection process 9 Specify the methods used to collect data from reports, including how many reviewers collected data from each report, whether they worked independently, any processes for obtaining or confirming data from study investigators, and if applicable, details of automation tools used in the process.

7
Data items 10a List and define all outcomes for which data were sought. Specify whether all results that were compatible with each outcome domain in each study were sought (e.g. for all measures, time points, analyses), and if not, the methods used to decide which results to collect.

7-8
10b List and define all other variables for which data were sought (e.g. participant and intervention characteristics, funding sources).
Describe any assumptions made about any missing or unclear information.
7-8, Table  1 Study risk of bias assessment 11 Specify the methods used to assess risk of bias in the included studies, including details of the tool(s) used, how many reviewers assessed each study and whether they worked independently, and if applicable, details of automation tools used in the process. Table 3 Effect measures 12 Specify for each outcome the effect measure(s) (e.g. risk ratio, mean difference) used in the synthesis or presentation of results. Table 2 Synthesis methods 13a Describe the processes used to decide which studies were eligible for each synthesis (e.g. tabulating the study intervention characteristics and comparing against the planned groups for each synthesis (item #5)).

8,
7-8, Table  1 13b Describe any methods required to prepare the data for presentation or synthesis, such as handling of missing summary statistics, or data conversions. n.a.
13c Describe any methods used to tabulate or visually display results of individual studies and syntheses.
7-8, Table  2 13d Describe any methods used to synthesize results and provide a rationale for the choice(s). If meta-analysis was performed, describe the model(s), method(s) to identify the presence and extent of statistical heterogeneity, and software package(s) used.
13e Describe any methods used to explore possible causes of heterogeneity among study results (e.g. subgroup analysis, metaregression). n.a.
13f Describe any sensitivity analyses conducted to assess robustness of the synthesized results. n.a.

Reporting bias assessment
14 Describe any methods used to assess risk of bias due to missing results in a synthesis (arising from reporting biases). 8

Certainty assessment
15 Describe any methods used to assess certainty (or confidence) in the body of evidence for an outcome. 8

Study selection
16a Describe the results of the search and selection process, from the number of records identified in the search to the number of studies included in the review, ideally using a flow diagram. Figure 1 16b Cite studies that might appear to meet the inclusion criteria, but which were excluded, and explain why they were excluded.

Figure 1
Study characteristics 17 Cite each included study and present its characteristics. 9, Table 1 Risk of bias in studies 18 Present assessments of risk of bias for each included study. 10, Table  3 Results of individual studies 19 For all outcomes, present, for each study: (a) summary statistics for each group (where appropriate) and (b) an effect estimate and its precision (e.g. confidence/credible interval), ideally using structured tables or plots.
10-11, Table 2 Results of syntheses 20a For each synthesis, briefly summarise the characteristics and risk of bias among contributing studies.

10-11
20b Present results of all statistical syntheses conducted. If metaanalysis was done, present for each the summary estimate and its precision (e.g. confidence/credible interval) and measures of statistical heterogeneity. If comparing groups, describe the direction of the effect.
10-11, Table 2 20c Present results of all investigations of possible causes of heterogeneity among study results.
n.a 20d Present results of all sensitivity analyses conducted to assess the robustness of the synthesized results. n.a.

Reporting biases
21 Present assessments of risk of bias due to missing results (arising from reporting biases) for each synthesis assessed. Table 3 Certainty of evidence 22 Present assessments of certainty (or confidence) in the body of evidence for each outcome assessed.

Discussion
23a Provide a general interpretation of the results in the context of other evidence.

11-14
23b Discuss any limitations of the evidence included in the review. 14 23c Discuss any limitations of the review processes used. 14 23d Discuss implications of the results for practice, policy, and future research.

OTHER INFORMATION Registration and protocol
24a Provide registration information for the review, including register name and registration number, or state that the review was not registered.
5 24b Indicate where the review protocol can be accessed, or state that a protocol was not prepared. 1 24c Describe and explain any amendments to information provided at registration or in the protocol. n.a.

Support
25 Describe sources of financial or non-financial support for the review, and the role of the funders or sponsors in the review. 1

Availability of data, code and other materials
27 Report which of the following are publicly available and where they can be found: template data collection forms; data extracted from included studies; data used for all analyses; analytic code; any other materials used in the review.